1. Field of the Invention
The present invention relates to a member for a push button switch that is suitable for a push button switch of electronic devices, and a method of manufacturing the member.
2. Description of the Related Art
In recent years, reduction in size and thickness has been required for electronic devices such as mobile phones and personal digital assistants (PDAs). Therefore, reduction in size and thickness has also been required for members for push button switches, which are used in operational portions of the electronic devices. Accordingly, a distance between upper and lower surfaces of a member for a push button switch is considerably reduced. For this reason, a metal frame, which has a smaller thickness and higher rigidity as compared to a resin frame, has been used.
A member for a push button switch needs to have a small thickness and high rigidity. Accordingly, a member for a push button switch used in a push button switch of electronic devices is for example, manufactured using the following method. First, through-holes and/or notches are formed in the surface of a hard base, which is formed of a rigid plate such as a hard resin plate or a metal plate. Then, the through-holes and/or notches are plugged using pad members made of rubber or a thermoplastic elastomer. Therefore, a keypad where pressing and floating portions of the pad members are integrated is manufactured. Subsequently, a key top is disposed on the surface of each pad member opposite to the pressing portion through an adhesive layer interposed therebetween. In this way, the member for the push button switch is manufactured (for example, refer to International Patent Publication No. WO2004/112069).
Further, there has been known a member for a push button switch manufactured using the following method. First, a reinforcing member is made using a hard resin by a die forming such as an injection molding, and a metal layer is formed on a portion of the reinforcing member, which comes in contact with an elastic member. Subsequently, the reinforcing member having the metal layer is moved into a cavity of a mold, and silicone rubber forming the elastic member is injected to perform a die forming. Accordingly, a base sheet where the elastic member and reinforcing member are integrated is obtained. Subsequently, a key top is fixed to the base sheet through an adhesive layer. As a result, the member for the push button switch is manufactured (for example, JP-A-2005-190849).
However, the conventional member for the push button switch has the following problems. In the case of the member for the push button switch disclosed in International Patent Publication No. WO2004/112069, the pad member having the floating portion to which the key top is fixed is mainly made of the single substance of silicone rubber or a thermoplastic elastomer. When the pad member is made of the simple substance of silicone rubber and the thickness of the floating portion is too small (for example, 0.2 mm or less), there is a problem that the floating portion is broken if the key top of a push button is pulled by mistake. Further, when the pad member is made of the simple substance of a thermoplastic elastomer having high strength, for example, the simple substance of a urethane-based elastomer, it is possible to make a key pad thinner. When the pad member is made of the thermoplastic elastomer, the pad member is generally manufactured using an injection molding. However, the flow property of the thermoplastic elastomer is poor as compared to that of liquid silicone rubber. For this reason, there are problems that the dimensional stability, which is required to form a thin film, of the thermoplastic elastomer is poor and yield is low. In addition, when the operation of the push button is repeated for a long term, the key pad is deformed due to a pressing force. For this reason, there is a problem that click feeling deteriorates.
Meanwhile, the member for the push button switch disclosed in JP-A-2005-190849 also has the same problems as the member for the push button switch disclosed in International Patent Publication No. WO2004/112069. Accordingly, it is difficult to reduce the thickness of the member for the push button switch. In addition, when the base sheet is formed, the reinforcing member having a previously-molded metal layer is disposed in the cavity of the mold and then needs to be molded again. For this reason, it takes a lot of time. Therefore, there are problems that manufacturing processes become troublesome, manufacturing cost increases, and productivity deteriorates.